[time-nuts] an interesting problem
jimlux at earthlink.net
Sat Feb 5 15:25:42 UTC 2011
Here's an interesting problem..
I've got a system at work with an internal clock oscillator that I want
to get some statistics on, but there's no direct visibility for the
oscillator, nor do I have a convenient test point that I can probe.
I can divide it down by an arbitrary number to generate pulses which I
then send out via SpaceWire timecodes. SpaceWire is a fast point to
point digital data link and it has a special capability that essentially
has a "tick in" signal at one end and a "tick out" signal at the other
end. The latency between tick in and tick out is random, but bounded
and discrete. the link runs at a clock rate derived from the same
oscillator, and you have to wait until the current character being sent
has been clocked out before you can send the special "timecode" token.
THat is, I can detect the "tick out" pulse and it has a random N*[0-14]
clock delay (distributed more towards 0 than 14) from when the "tick in"
(which is synchronous with the clock I want to measure). N is the ratio
between my clock and the data rate on the wire ( 7, in this case, so the
time step is about 100ns)
So, by making measurements of the time when the "tick out" appears (or
time between tick_out pulses) can I somehow "take out" the random
variability of the link.
It seems, since the clock isn't *terrible* that I could, for instance,
accumulate statistics, and throw out the ones that have more than 0
clock latency (which is probably a few 10s of percent of the ticks.. I
haven't looked yet). Or, given that the interval between ticks is one
of 28 or 29 discrete values (plus the underlying clock variance), if the
clock variance on a given pulse is <<clock rate, the
histogram/probability distribution of times would look like a bunch of
little humps, each with variance =clock variance.
or, is a real oscillator going to have an instantaneous variance that is
comparable to or greater than 1 clock pulse?
tvb's site has some sample ADEV numbers, and I would imagine that his
HTV-2 TCXO has comparable performance (at least over the short run at
constant temperature).. He shows ADEV of 1E-9 from 1-1000 seconds...
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